Information Notice 2006-31, Inadequate Fault Interrupting Rating of Breakers
| ML063000104 | |
| Person / Time | |
|---|---|
| Site: | Point Beach, Cook  |
| Issue date: | 12/26/2006 |
| From: | Michael Case NRC/NRR/ADRA/DPR |
| To: | |
| Carla P. Roquecruz, 301-415-1455 | |
| References | |
| IN-06-031 | |
| Download: ML063000104 (4) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
December 26, 2006
NRC INFORMATION NOTICE 2006-31:
INADEQUATE FAULT INTERRUPTING RATING
OF BREAKERS
ADDRESSEES
All holders of operating licensees for nuclear power reactors, except those who have
permanently ceased operations and have certified that fuel has been permanently removed
from the reactor vessel.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform
addressees about the concerns associated with inadequate fault interrupting rating of breakers
at Point Beach Nuclear Plant (PBNP) and D.C. Cook nuclear plants. It is expected that
recipients will review the information for applicability to their facilities and consider actions, as
appropriate, to avoid similar concerns. However, suggestions contained in this information
notice are not NRC requirements; therefore, no specific action or written response is required.
DESCRIPTION OF CIRCUMSTANCES
In the Event Notification42020 dated September 27, 2005, the Nuclear Management
Company (NMC) identified that, in the electrical distribution system of its PBNP, the postulated
three-phase fault currents exceed the maximum listed interrupting rating of circuit breakers and
associated bus bar bracing rating at some of the 13.8 kV, 4.16 kV, and 480 V switchgear
(includes power panels, motor control centers etc.). Relating to this EN, NMC also issued a
licensee event report (LER) # 266/2005-005 dated November 18, 2005, which described the
condition as Operable but potentially Nonconforming with respect to Title 10 of the Code of
Federal Regulations (10 CFR) Appendix R requirements. Inadequate capacity of 4.16 kV
breakers/switchgear to withstand three-phase bolted fault was also reported at the D.C. Cook
Nuclear Plant in the NRC Inspection Report 50-315/316/01-19 (Agencywide Documents Access
and Management System (ADAMS) Accession No.: ML020230387).
DISCUSSION
LER # 266/2005-005 referred to LER # 266/97-032, dated July 30, 1997, which described the
underlying condition and stated the issues identification in 1993. The short circuit current
calculation in 1993 concluded that under specified plant conditions, at certain 480 V, 4.16kV,
and 13.8 kV switchgear buses, the postulated bolted three-phase symmetrical fault currents
may be higher than the capability of the equipment involved. The root cause of the problem
was the design of the switchgear based on initial short circuit calculations which did not take
into account the contributions of the motor currents. In general, if a fault is considered downstream of a load breaker, on a given switchgear bus, the electrical motors fed from the
same bus can contribute to the fault current significantly, typically six times the motor rated
current; motors act as generators during the fault duration. Both safeguards and
non-safeguards buses were potentially affected by this condition. However, based on the low
probability for an actual bolted fault, the judgment in 1993 was that actual cable impedance
would limit any fault current to a value below the equipment rating.
In 2004, NMC performed a revision of the Short Circuit and Degraded Voltage Analysis for the
PBNP. The results of the short circuit calculation reaffirmed that a large number of 13.8 kV,
4.16 kV, and 480 V switchgear had calculated short circuit currents greater than the equipment
rating. The results did not meet the acceptance criteria described in the above analysis, which
stated that Electrical plant equipment shall be rated to withstand and properly interrupt the
largest credible short circuit current available at the specified equipment location for licensed
plant alignments. NMC implemented interim compensatory measures, such as twice-per-shift
fire watches in applicable fire-zones. The long term measures, such as replacement of
breakers, bracing of bus bars, transformer tap setting changes, etc., are also in progress. The
4.16 kV breakers are being replaced at D.C. Cook nuclear plant to resolve the issue of
inadequate interrupting rating of breakers.
It is important that electrical switchgear is adequately rated not only for the normal operating
current, but also able to safely interrupt a three-phase bolted fault, even though the probability
of such a fault is very low. An inadequately rated breaker can fail or destroy itself in trying to
open a three-phase bolted fault, resulting in a secondary fire at the switchgear. According to
the NRC research report titled Operating Experience Assessment, Energetic Faults in 4.16 kV
to 13.8 kV Switchgear and Bus Ducts That Caused Fires in Nuclear Power Plants 1986-2001"
(Agencywide Documents Access and Management System Accession No. ML021290358), the
consequences of an electrical fire (especially at medium voltage) can be wide-ranging, with the
potential to damage an entire line of switchgear. National Electric Code, Section 110-9 states, Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient
for the nominal circuit voltage and the current that is available at the line terminals of the
equipment. In general, the industry practice has been to provide switchgear adequately rated
to withstand and safely interrupt a three-phase bolted fault current. It is also important that
voltage drop and short circuit calculations are updated after each significant electrical plant
modification and the switchgear is adequately rated to withstand and safely interrupt an
appropriate fault current.
RELEVANT GENERIC COMMUNICATIONS
Generic Letter 88-15, Electric Power Systems - Inadequate Control Over Design Processes, also emphasized the issue of inadequate fault current interrupting capability of the breakers.
CONTACT
This information notice requires no specific action or written response. Please direct any
questions about this matter to the technical contact listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.
/RA by TQuay for/
Michael J. Case, Division Director
Division of Policy and Rulemaking
Office of Nuclear Reactor Regulation
Technical Contacts: Vijay Goel, NRR
301-415-3730
E-mail: vkg@nrc.gov
Carla Roquecruz, NRR
301-415-2018
E-mail: cpr1@nrc.gov
Note: NRC generic communications may be found on the NRC public Website, http://www.nrc.gov, under Electronic Reading Room/Document Collections.
ML063000104 OFFICE
EEEB:DE
Tech Editor
OIEB:DIRS
BC:EEEB:DE
TL:IOEB:DIRS
NAME
V Goel
CClark(by e-mail)
CRoquecruz
IJung
JThorp
DATE
11/06/2006
10/23/2006
11/07/2006
11/06/2006
11/07/2006 OFFICE
LA:PGCB
PGCB:DPR
BC:PGCB:DPR
D:DPR
NAME
CHawes
QNguyen
CJackson
MCase
DATE
12/07/2006
12/07/2006
12/22/2006
11/26/2006